In the lithographic printing process, a lithographic plate comprising an aluminum sheet is initially coated with a photosensitive layer usually containing, among other things, one or more resins. A negative is applied over the photosensitive layer, and the two are exposed to high intensity light. The negative is then removed, and the plate is processed to develop it. In development, the image areas of the photosensitive layer are retained on the plate surface and the non-image areas are removed. Although some manual developing is still done, the large scale commerical use of lithographic plates has made automatic processing of necessity. Numerous apparatus have been proposed to perform such processes. In general, the more successful machines have emulated the manual development procedures by subjecting the plate to a scrubbing action while flooding the plate with developer liquid and then flooding the plate with gum arabic. However, there has always been need to improve the apparatus to perform tasks more satisfactorily.
In the developing step, the developer liquid becomes dirty as it removes the unimaged portions of the photosensitive coating. After the developer liquid becomes dirty, any further scrubbing or permitting the dirty developer liquid to remain on the plate can result in random redeposition of the removed coating material on the plate, causing obvious image defects in the developed plate. To avoid this, prior art apparatus typically remove the dirty developer liquid from the plate while scrubbing, or immediately thereafter, and reapply the developer liquid one or more times. The advantages of shortcutting this process can be easily appreciated.